Plague proportions of mice in parts of South Australia are providing researchers with the opportunity to better understand the factors driving changes in population
A combination of live-trapping, kill-trapping and active-burrow counts are being used by Julianne Farrell, on behalf of the Invasive Animals Cooperative Research Centre, as part of the work to better predict changes in mouse populations.
PHOTO: Emma Leonard
In the past 20 years the frequency of mouse plagues has increased from one in every five to six years to one in every four years. Intensive cropping rotations, conservation farming practices and higher yields are all considered to be contributing factors.
However, current modelling of mouse population dynamics is less than perfect. The Invasive Animals Cooperative Research Centre (IACRC), which is supported by the GRDC, is continuing to investigate what drives mouse populations to plague levels and how to better monitor for early indicators of high plague risk. It is also working on better management of mice populations using integrated pest management approaches.
“Our objective is to better predict increases in mice activity and improve on-ground management. Other investment is focused on new poisons and antidotes and their targeted delivery,” says Dr Simon Humphrys, IACRC program leader.
Dr Humphrys was one of the presenters at a mouse forum held in Maitland, South Australia, before seeding began.
Maitland grower Ben Wundersitz reported seeing mouse damage in windrowed crops in November 2013. Since then, mouse numbers on land he farms, which runs across the width of the Yorke Peninsula, have continued to increase.
Mr Wundersitz has a clearer picture of mouse numbers in his paddocks as they have been included in a mouse population survey carried out by Julianne Farrell for the IACRC.
A combination of live-trapping, kill-trapping and active-burrow counts have indicated mouse numbers of more than 1000 per hectare in some areas of Yorke Peninsula.
“Marking burrows with talc and monitoring if the talc is disturbed overnight is an accurate and simple method of monitoring mouse activity before seeding,” Ms Farrell says.
Her estimations of mouse populations are based on a statistical model that uses capture-mark-recapture trapping data.
Feed is in plentiful supply after good yields in the west of SA in 2013. In addition to the usual behind-the-harvester grain loss estimates of about two per cent per tonne, parts of the region also suffered pre-harvest grain loss due to an extreme wind event.
Hygiene and baiting
Best-practice control was also discussed at the mouse forum held in Maitland, South Australia, in early April. Hygiene and baiting remain the key control options. Growers should clean up spills of grain in the paddock and around yards, and remove rubbish and nesting material from sheds.
Where mouse numbers are high in autumn, baiting several weeks prior to seeding, combined with baiting within 24 hours of seeding, is considered to be the most appropriate strategy.
Trials in Queensland have found much lower kill rates when baiting in-crop. In maturing crops of wheat and sorghum, bait efficacy was between 40 and 52 per cent compared with nearer 90 per cent pre-sowing.
This was with a rate of one kilogram of bait per hectare applied with mouse densities equivalent to 266 to 1066/ha.
Kill rates did not vary with mouse density. Ideally, mouse bait should be used in dry conditions and applied late in the day to achieve maximum ingestion by mice and not birds. Zinc phosphide baits will tolerate some rain, but it will reduce their effectiveness. Rebaiting may be required if rainfall occurs within the first few days of baiting.
To manage mice during harvest growers should:
minimise grain loss, harvest before crops are overripe and pod shatter or grain loss occurs;
monitor and minimise grain loss with sieve settings and harvester speed;
not leave strips of unharvested grain; and
avoid and clean up any grain spills and thoroughly clean machines before storing.
The survey included wheat, barley and canola stubbles and while there were no clear patterns for crops or soils, fewer mice were recorded in stubble paddocks that had been grazed.
Ms Farrell puts this down to the mice competing for feed and reduced shelter from predators.
GRDC Southern Panel member Bill Long, who facilitated the meeting at Maitland, says growers are aware of the potential for mouse numbers to increase rapidly and have been on alert for potential problems since 2010, when they reached plague proportions.
“No-till conservation cropping, increased cropping frequency and improved yields are all potential factors contributing to continued higher populations,” he says.
However, establishing mouse populations is complicated by the fact that mice can move several hundred metres, mainly at night. This can affect both counting and baiting.
To counter this, Ms Farrell used a system of live traps set in a grid at 10-metre intervals. All mice caught were marked by clipping fur and released after the first two nights of trapping, then killed and necropsied to gather breeding data on the third morning. Population estimates are based on the recapture numbers. Low recapture levels indicate higher mouse populations.
Traps were set in paddocks as well as in native vegetation corridors where high mouse numbers create the potential for re-invasion into paddocks after baiting.
In addition to collating live mouse numbers, breeding data was gathered with necropsy of trapped females. Past pregnancies were estimated from the number of uterine embryo scars, while the number of fertilised embryos and stage of development was also recorded.
A further part of the monitoring program looked at the impact of mouse baiting. Even at these high mouse numbers, the registered label rate of one kilogram/ha of zinc phosphide bait was achieving high knockdown rates.
Past research has shown that higher rates (2kg/ha) do not work any better. Repeated application of 1kg/ha provides the best control for re-invading mice.
0411 257 831,
Dr Simon Humphrys,
0428 225 530,
For related GRDC resources go to Ground Cover Direct
Digital mouse monitor
Testing urged for mite resistance
GRDC Project Code